1. Field of the Invention
This invention relates to an apparatus for measuring particles in a fluid, more particularly to an apparatus for measuring the diameter and concentration of particles in a fluid by the photon counting method.
2. Description of the Prior Art
In conventional apparatuses of this type, the fluid containing particles to be measured is placed in a measurement cell and irradiated with a laser beam, and the characteristics of the particles are determined from the light scattered by the particles using a photomultiplier.
The light scattered by a particle in the fluid becomes weaker in proportion as the diameter of the particle becomes smaller, and it is known that in carrying out detection with respect to weak light, it is more effective to use the photomultiplier in accordance with a photon counting method than to use it in an analogue manner.
The conventional apparatus will first be explained with reference to FIGS. 4 and 5.
In FIG. 4, a laser beam from a laser beam source 1 is condensed on a measurement region 3 by a lens 2. The measurement region 3 is within a measurement cell 4 through which a fluid flows. When particles pass through the measurement regions, they scatter the incident laser light. Light scattered by the particle is focused by a lens 5 to form an image at a slit 6, and the light passing through the slit 6 advances to the photocathode of a photomultiplier tube 7 which serves as a photoelectric converter. This light can be considered to be constituted of particles, i.e. photons, and the photons reaching the photocathode cause electrons to be emitted from the same photocathode by the photoelectric effect. The electrons emitted from the photocathode are multiplied in number within the photomultiplier tube 7 by a factor of approximately 10.sup.6 and output as a signal from the photomultiplier tube. This output signal is amplified by a preamplifier 8 and then passed through a peak discriminator 9 which serves as a pulse generator. The threshold value of the peak discriminator 9 is set at the lower limit of the voltage or current value corresponding to the signal at the time when a single electron is emitted from the photocathode of the photomultiplier 7. When the signal amplified by the preamplifier 8 is greater than the threshold value of the peak discriminator 9, the peak discriminator 9 outputs a pulse. This pulse is sent to a pulse shaper 10 where it is shaped.
The shaped pulses output by the pulse shaper 10 are called "photoelectron pulses" and the method involving the counting of these photoelectron pulses is the "photon counting method." In the photon counting method, the number of photoelectron pulses per unit time is proportional to the intensity of the scattered light from the particle. Therefore, by counting the number of photoelectron pulses per fixed time interval, it becomes possible to measure the intensity of the scattered light from particle.
In FIG. 4, the photoelectron pulses output from the pulse shaper 10 are counted in a pulse counter 11. The pulse counter 11 is initialized by a reset signal output from a processor system. The count value of the pulse counter 11 is latched by a latch circuit 12 from which it is read by a processor 13. The latch circuit 12 is controlled by a read signal from the processor 13.
As shown in greater detail in FIG. 5A, this conventional apparatus consists of a single pulse counter 11, a latch circuit 12 for latching the count value of the pulse counter 11, and a processor 13 for reading the count latched in the latch circuit and for controlling the pulse counter and latch circuit.
As shown in FIG. 5B, this arrangement results in photoelectron pulses 15 being applied to a terminal 14 of the pulse counter 11 independently of and without relation to the read and reset signals output by the processor 13. When a predetermined period of time has lapsed after the start of pulse counting by the pulse counter 11, a read signal 16 is issued to cause the count value of the pulse counter 11 to be latched by the latch circuit 12. After the count value of the pulse counter 11 has been latched by the latch circuit 12, the pulse counter 11 is initialized by a reset signal 17. At this time, the pulse width T of the reset signal has to be made longer than the time constant determined by a resistor R and a capacitor C which are provided to prevent malfunction of the pulse counter 11 because of noise.
When the reset signal 17 becomes low level, the pulse counter 11 resumes the counting state and the counting of the photoelectron pulses resumes. The count value latched by the latch circuit 12 is read during the period that the read signal from the processor 13 is at high level. By repeating these operations, it is possible to count the number of photoelectron pulses within fixed time intervals.
When the photoelectron pulses are counted by the aforesaid conventional method, the processor 13 cannot read the photoelectron pulses 15 input to the pulse counter 11 during the time L indicated in FIG. 5B. This means that there is a blind period during which the photoelectron pulses are not counted, so that there arises an error in the measurement of the concentration of the particles in the fluid.